The condition of the muscles in the human body greatly affects a person's quality of life, efficiency at work, contentment and many other things. When measuring muscle condition, a good standard is a comparison with the general population of the same age. Muscle condition, that is the endurance and fatigue of muscles, can be determined using different kinds of tests, but those known in the art do not give direct and quick reliable results or reliable results for comparison, for instance, with the general population.
Muscles consist of muscle fibers, that is, muscle cells. A single nerve fiber regulates many muscle fibers. Muscle fibers divide into myofibrils and again into myofilaments which divide into actin and myosin filaments. Muscular contraction occurs mechanically in these micro units due to the entry of Ca++-ions with subsequent hydrolysis of adenosine triphosphate (ATP). Muscular contraction is caused by a nerve impulse which releases Ca++-ions from cytoplasm into myofibrils, and this causes actin and myosin filaments to slide into contact with each other. Simultaneously, the action potential proceeds on the muscle cell membrane, thus causing a parallel phenomenon in millions of actino-myosin units.
Many muscle cells form functional units which are called motor units. When muscle strength has to be increased, more motor units are recruited into the contraction process.
The action potentials involved can be measured and are measured electrically from inside the muscle cell, for instance by using needle electrodes (needle electromyography, needle EMG) or surface electrodes (surface EMG). Motor units, that is actino-myosin units, function at different frequencies and are stimulated at different times. Thus an EMG-signal of a contracting muscle, recorded for instance at the skin surface, contains frequency components mainly in the frequency range of 10-400 Hz. The intensity of the recording signal varies in this case from a few micro volts (.mu.V) to about 5000 .mu.V. This kind of measurement and observation of muscles is very common and the results are used for different purposes.
Muscular contraction obtains its energy from ATP. Oxygen is important for both synthesis of the ATP and its catabolism. During heavy muscle work, as a result of the breakdown of ATP, many hydrogen ions are formed inside the muscle cell which do not bind to other chemical reactions with oxygen. Thus the acidity of cytoplasm increases, the production of energy decreases and the muscle begins to fatigue. As the muscle cell becomes acidic, the conduction rate of the muscle cell membrane diminishes and the formation of an action potential in the muscle cell slows down and the penetration of nerve impulses into the muscle cell is inhibited. This phenomenon can be detected with present-day measuring devices.